It is known to provide blades with lightning protection systems to protect them from lightning strikes. As an example, WO 96/07825 discloses a lightning protection system, in which the tip of the blade is provided with a so-called lightning receptor made of an electrically conductive material. This lightning receptor can “capture” a lightning strike and conduct the current through a lightning down conductor, said conductor extending in the longitudinal direction of the blade and being earthed via the rotor hub of the wind turbine. This system has often been shown to provide a satisfactory protection against lightning strikes.
However, there is a risk of a flashover or sparkover from the lightning receptor or lightning down conductor to areas in the blade, where water has accumulated, since polluted water is electrically conductive. In this case, lightning current heats up the water, which may cause a “steam explosion”. This may result in pressure increases, which are sufficiently high to damage the blade. Flashovers may also occur because of deposits of, for example, particles from the air, salts or insects on the surface of the blades. The damages due to flashovers result in the blade having to be repaired or, in the worst case, exchanged, which is a time-consuming and expensive process, since it is complicated to repair or exchange blades, especially for offshore wind power plants.
Furthermore, there is a risk of striking the lightning down conductor (or flashovers) through the surface of the blade, which at best creates only a small hole through the surface of the blade, but often causes greater damage to the blade. For a better control of the lightning current through the blade without damage thereof, the lightning protection system may be provided with several lightning receptors (multi-receptors) or lightning down conductors along the longitudinal direction of the blade. Preferably, the lightning receptors are arranged at a maximum interval of five meters to ensure that there are no lightning strikes (or flashovers) through the surface of the blade. However, this is a comparatively expensive and complicated solution.
WO 2007/062659 provides a solution in which an inner lightning conductor is electrically insulated in its entire longitudinal extent.
US 2008/095624 discloses a rotor blade with a lightning protection system comprising an insulated down conductor, where a dielectric sheet is used as insulation.
WO 00/14405 discloses a wind turbine blade, wherein the blade comprises a blade wall including oblong strips containing carbon fibre-reinforced plastics, which are electrically connected to an inner lightning down conductor.
EP 1 011 182 discloses a wind turbine blade provided with an electrically conductive tape layer on the outer surface.
WO 01/77527 discloses a wind turbine blade comprising a lightning protection system comprising one or more internal conducting means and one or more external lightning conducting means.
WO 2008/006377 discloses a wind turbine comprising a nacelle and at least one wind turbine blade, wherein at least one of said parts comprises a conductive film layer with connection to a ground potential.
DE10228665, DE10233528 and WO 2006/099880 describe external lightning protection systems provided with a lightning down conductor and a connection piece adapted for preventing sliding discharges. The lightning down conductor has an electrical conductor embedded in an insulating casing, which in turn may be coated with a conductive layer having a resistance of up to 1 Mega Ohm per meter.
WO 98/18186 discloses a lightning conductor comprising an inner electrical conductor, an insulated layer surrounding the inner electrical conductor, and a resistive semiconductive layer surrounding the insulating layer. The semi-conductive layer has a resistance falling within the range from about 100 ohms per meter to about 100,000 ohms per meter.
U.S. Pat. No. 4,687,882 discloses a shielded power cable comprising inner and outer conductors separated by cable insulation defining a displacement current path between the conductors for high frequency currents.